With rapid development of information communication technology in recent years, we are moving toward a ubiquitous society based on the information communication technology.
In order to access information communication devices anytime anywhere, it is necessary to install sensors each having a computer chip capable of performing a communication function in all social facilities. Consequently, the supply of power to the devices and sensors is a new issue. In addition, as kinds of portable devices, such as a mobile phone, a Bluetooth headset, and an iPod music player, have been increased, it takes time and efforts for users to charge batteries. In recent years, wireless power transfer has attracted attention as a method of solving this problem.
Wireless power transmission or wireless energy transfer refers to a technology of wirelessly transferring electric energy from a transmitter to a receiver using a principle of magnetic induction. In the 1800's, an electric motor or a transformer employing a principle of electromagnetic induction had been used, and afterward a method for transmitting electric energy by emitting electromagnetic waves, such as radio waves or lasers, has been attempted. Actually, electrical toothbrushes or electrical razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction.
Wireless energy transfer schemes used until now may be divided into a magnetic induction scheme, an electromagnetic resonance scheme, and a power transfer scheme using a short-wavelength radio frequency.
The magnetic induction scheme is a technology using a phenomenon in which when two coils are disposed adjacent to each other and current is supplied to one of the coil, a magnetic flux is generated and, as a result, electromotive force is generated in the other coil. The magnetic induction scheme is being rapidly commercialized for small-sized devices, such as a mobile phone. The magnetic induction scheme has an advantage in that it is possible to transfer power of a maximum of several hundreds of kilowatts (kW) with high efficiency. However, the magnetic induction scheme has a disadvantage in that the maximum transfer distance is 1 cm or less and thus coils must be disposed adjacent to a charger or the bottom of a mobile phone.
The electromagnetic resonance scheme is characterized by using an electric field or a magnetic field instead of utilizing an electromagnetic wave or current. The electromagnetic resonance scheme has an advantage in that the electromagnetic resonance scheme is not affected by the electromagnetic wave and is thus safe for other electronic devices and human bodies. However, the electromagnetic resonance scheme has a disadvantage in that the electromagnetic resonance scheme is utilized within a limited distance and space and energy transfer efficiency is somewhat low.
The short-wavelength wireless power transfer scheme, simply an RF scheme, utilizes that energy can be directly transmitted and received in a radio wave form. This technology is an RF wireless power transfer scheme using a rectenna. Rectenna, which is a compound word of an antenna and a rectifier, means an element for directly converting RF power into DC power. That is, the RF scheme is a technology of converting an AC radio wave into DC. In recent years, research has been actively conducted on commercialization of the RF scheme with improvement in efficiency of the RF scheme.
Wireless power transfer may be variously utilized in various industries, such as IT, railway, and electric home appliance industries as well as a mobile industry.
Particularly, magnets may be provided in a wireless power transmitter and a wireless power receiver such that the transmitter can sense the receiver. The transmitter may sense a magnetic field of the receiver to determine whether to supply power to the receiver.
Conventionally, however, it is not possible for the transmitter to effectively sense the magnetic field of the receiver.